1. Field of the Invention
The present invention relates to a driving force transmission system for an engine.
2. Background Art
Conventionally, an intake camshaft and an exhaust camshaft of an engine are connected to a crankshaft via a power transmission mechanism constituted by a timing chain, sprockets, and the like. The timing chain receives an excitation force by reciprocal motion of an intake valve, an exhaust valve, and the like.
When a vibrational frequency of an excitation force approaches a natural frequency (resonant frequency) of a power transmission mechanism, a timing chain is brought to a resonant state, and an excessive tension force is generated in the timing chain. As a result, the life of the timing chain may be shortened.
In order to eliminate the aforementioned drawback, for instance, Japanese Unexamined Patent Publication No. 2011-43092 discloses the following configuration. Specifically, in the aforementioned publication, a plunger of a fuel pump connected to an exhaust camshaft is caused to reciprocally move with a phase opposite to the phase of an exhaust valve in reciprocal motion. According to the aforementioned configuration, an excitation force by the exhaust valve is offset by an excitation force by the fuel pump.
In recent years, development on an engine having a high compression ratio has progressed as one of the measures for improving fuel economy of the engine. In such an engine, it is necessary to finely control a valve timing. In view of the above, an intake cam and an exhaust cam are controlled to open and close at an optimum timing by a variable valve timing mechanism (VVT). A camshaft, and a sprocket for transmitting a rotational force of a crankshaft to the camshaft are connected to each other via the VVT. Further, a high-pressure fuel pump may be used to finely control fuel injection into a combustion chamber.
In a general engine, a fuel pump is configured such that a fuel pump drive shaft is connected to an end of a camshaft, and the fuel pump is driven by a rotational force of a crankshaft together with the camshaft via a timing chain or a timing belt. In order to drive a high-pressure fuel pump, a large driving force is necessary. Therefore, when a large driving force is transmitted to a camshaft via a VVT, particularly, when a VVT is an electric VVT configured to be driven by a motor, a torque necessary for driving a high-pressure fuel pump may exceed a torque to be generated by the motor of the electric VVT. As a result, the VVT may not be operated. In view of the above, it is not appropriate to connect a high-pressure fuel pump to a camshaft when a VVT is mounted on the camshaft. As well as the aforementioned configuration, when a VVT is hydraulically operated, a high hydraulic pressure is necessary so that the VVT is operated with a torque larger than a driving torque of a fuel pump. This may lead to an increase in load of an oil pump, and an increase in mechanical resistance.
In view of the above, the inventor conceived an idea of operating a high-pressure fuel pump by disposing a drive shaft (pump drive shaft) of the high-pressure fuel pump between a crankshaft and a camshaft in the height direction of an engine, mounting two sprockets on the pump drive shaft, winding a first timing chain around one of the sprockets and a sprocket of the crankshaft, and winding a second timing chain around the other of the sprockets, an intake camshaft, and an exhaust camshaft.
However, the aforementioned configuration is not a configuration as disclosed in the aforementioned publication, in which a fuel pump is connected to an exhaust camshaft. Therefore, it may be impossible to offset an excitation force by reciprocal motion of an exhaust valve by an excitation force by a fuel pump. As a result, it may be impossible to suppress an excitation force acting on a timing chain. This may cause a phenomenon that the timing chain is brought to a resonant state.